Powerful billionaire shareholders at the pub restaurant group Mitchells & Butlers have been blocking the appointment of independent non-executive directors, prompting the departure last week of two directors, including the chairman, Simon Burke.

Bob Ivell, who has taken over as interim chairman, will come under intense scrutiny this week over M&B's failure to scotch fears among small shareholders that the board, depleted and in disarray, is being dominated by a handful of billionaire investors led by Joe Lewis and the Irish investors JP McManus and John Magnier.

The Guardian has learned that, over several months, a number of candidates to join the board as independent non-executive directors – looking out for the interests of all shareholders – were blocked by M&B's powerful investors or their representatives in the boardroom.

Failure to bring sufficient new blood on to the board over the last 18 months is thought to have been a large part of the reason the former Hamley's boss Simon Burke quit as chairman last week, along with fellow director Mike Balfour.

John Lovering, Burke's predecessor at the pub group behind O'Neill's, Toby Carvery and All Bar One, threw in the towel in February after just 12 months in the job, also feeling frustrated his efforts to repair fraught relations with institutional shareholders had been hampered.

Ivell, the sixth person to chair M&B in four years, will deliver a trading update on Thursday alongside the interim chief executive, Jeremy Blood, another caretaker appointment forced on the board by an exodus of directors unhappy with the level of influence being wielded by two big shareholders. His predecessor, Adam Fowle, who joined the pub group in 1984, quit in March.

As well as interim chairman and chief executive, the depleted M&B board now consists of the finance director Tim Jones, who has only been at the company since October last year, and two representatives of the major shareholder Piedmont. Trying to help the company reverse the boardroom exodus is James Hyde, a partner at the recruitment firm Korn/Ferry Whitehead Mann.

The latest round of boardroom departures has reawakened fears that Piedmont and Elpida – respectively holding 22.7% and 20.1% of M&B shares – are determined to ride roughshod over the kind of corporate governance checks and balances small investors look for in order to ensure their interests are protected.

Piedmont is the investment vehicle of the Bahamas-based billionaire currency trader Joe Lewis, while Elpida is a vehicle controlled by Lewis's friends and occasional business associates, JP McManus and John Magnier.

At the end of 2008 M&B's then board, led by chairman Simon Laffin, voted to remove two representatives of Piedmont and two other directors – suspected of being close to Elpida – from the board. Laffin accused the leading shareholders of covertly attempting to bully independent directors and in effect control the business even though they only held about 40% of shares.

At a stormy shareholder meeting the following January, Laffin himself faced a shareholder vote to stay on the board. Despite winning near-unanimous support from institutional investors, he was ousted with other independent directors by the voting might of Elpida and Piedmont. In their place were installed four directors nominated by Lewis – Lovering, Burke, Balfour and Blood – all of whom nevertheless claimed to be independent, promising to restore minority investor confidence in the board. Before their election to the board, the four issued a joint statement insisting that they were not stooges for Lewis.

"We have made it clear from the outset that we are standing as independent directors and, if elected, would seek to act in the interests of all shareholders," they said. "Our intention in standing for election was to try to break the logjam in the relationship between the board and certain shareholders, and to bring a new independent perspective to bear on how shareholder value in M&B can best be grown."

With three of these four having quit, concerns that the company is dominated by Lewis and his friends has again come to the fore. Ivell is the only new non-executive director to emerge in 18 months, despite repeated promises that further appointments were imminent. He is described by M&B as "independent", but he did spend two years as deputy chairman of the racquets club chain Next Generation at a time when it was owned by a group of wealthy Irish investors including McManus and Magnier.

In May this year M&B told investors it had ditched its long-standing auditors Ernst & Young, upon whom shareholders large and small had been depending to ensure a true and fair set of accounts. In fact an official E&Y resignation letter was dated 23 March. It said: "The [M&B] directors decided to retender the audit and have chosen to appoint Deloitte as the company's new auditors. Ernst & Young decided not to participate in this process as we believed it unlikely that we would be successful."

Other advisers ditched in the last 18 months include law firm Allen & Overy, investment banking adviser Citigroup and joint house broker Merrill Lynch. Meanwhile below board level there have also been extensive changes. Of the newly restructured ten-seat executive board just four have been at M&B longer than nine months. As well as Blood and Jones, new to M&B executive roles are Gary John, property director, Robin Young, commercial director, Doug Evans, general counsel and Saudagar Singh, head of human resources.

Since the departure of Fowle, the smooth running of M&B is understood to have become increasingly dependent on three experienced divisional heads Kevin Todd, Amanda Coldrick and Roger Moxham.

Asked before the last week's boardroom bloodletting if he had received inquiries from M&B managers wishing to defect, the chief executive of one rival operator said: "Absolutely, I am. It's incredible. They're banging down my door."